An experiment left on the lunar surface 30 years ago by the
Apollo 11 astronauts continues to return valuable data about the
Earth-Moon system to scientific centers around the world,
including NASA's Jet Propulsion Laboratory, Pasadena, Calif.

Scientists who analyze the data from the Lunar Laser Ranging
Experiment have measured, among other things, that the Moon is
moving away from the Earth and that the shape of the Earth is
changing. They have also used
the experiment to test the validity of several predictions of
Einstein's Theory of Relativity.

The lunar laser ranging reflector is designed to reflect
pulses of laser light fired from the Earth. The idea was to
determine the round-trip travel time of a laser pulse from the
Earth to the Moon and back again, thereby calculating the
distance between the two. Unlike the other scientific
experiments left on the Moon, this reflector requires no power
and is still functioning perfectly after 30 years.

The reflector consists of a checkerboard mosaic of 100 fused
silica half cubes (roughly the size of the average computer
monitor screen), called corner cubes, mounted in a 46-centimeter
(18-inch) square aluminum panel. Each corner cube is 3.8
centimeters (1.5 inches) in diameter. Corner cubes reflect a
beam of light directly back toward the point of origin; it is
this fact that makes them so useful in Earth surveying.

"The Lunar Laser Ranging project cuts across disciplinary
and international boundaries, measuring characteristics of the
Earth, the Moon and gravitational physics," said Dr. James
Williams, a research scientist at JPL. "Data analysis has been
conducted around the world, including Germany, France and the
U.S."

The McDonald Observatory Laser Ranging Station near Ft.
Davis, Texas, and the Observatoire de la Cote d'Azur, operated by
the Centre de Recherche en Geodynamique et Astrometrie
near Grasse, France, regularly send a laser beam through an
optical telescope and try to hit one of the reflectors. The
reflectors are too small to be seen from Earth, so even when the
beam is correctly aligned in the telescope, actually hitting a
lunar reflector is quite challenging. At the Moon's surface the
beam is roughly one mile wide; scientists liken the task of
properly aiming the beam to using a rifle to hit a moving dime
two miles away.

Once the laser beam hits a reflector, scientists at the
observatories use sensitive filtering and amplification equipment
to detect any kind of return signal. The reflected light is too
weak to be seen with the human eye, but, under good conditions,
one photon -- the fundamental particle of light -- will be
received every few seconds.

Three more reflectors have since been left on the Moon,
including two by later Apollo missions and one (built by the
French) by the unmanned Soviet Lunakhod 2 lander. Each of the
reflectors rests on the lunar surface in such a way that its flat
face points toward the Earth.

Continuing improvements in lasers and electronics over the
years have lead to recent measurements that are accurate to about
two centimeters (less than one inch). Scientists know the
average distance between the centers of the Earth and the Moon is
385,000 kilometers (239,000 miles), implying that the modern
lunar ranges have relative accuracies of better than one part in
10 billion. This level of accuracy represents one of the most
precise distance measurements ever made and is equivalent to
determining the distance between Los Angeles and New York to one-
hundredth of an inch.

During the course of the last 30 years, scientists have been
able to use the orbit of the Moon and the data they received
through lunar ranging to study events happening on Earth.

There have been major scientific advances derived from lunar
ranging:

The familiar ocean tides raised on the Earth by the Moon
have a direct influence on the Moon's orbit. Laser ranging has
shown that the Moon is receding from the Earth at about 3.8
centimeters (1.5 inches) every year.

Lunar ranging, together with laser ranging to artificial
Earth satellites, has revealed a small but constant change in the
shape of the Earth. The land masses are gradually changing after
being compressed by the great weight of the glaciers in the last
Ice Age.

Predictions of Einstein's theory of relativity have been
confirmed using laser ranging.

Small-scale variations in the Moon's rotation have been
measured. They result from irregularities in the lunar gravity
field, from changes in the Moon's shape due to tides raised in
the Moon's solid body by the Earth and from the effects of a
fluid lunar core.

The combined mass of the Earth and Moon has been determined
to one part in 200 million.

Lunar ranging has yielded an enormous improvement in our
knowledge of the Moon's orbit, enough to permit accurate analyses
of solar eclipses as far back as 1400 BC.

The atmosphere, tides and the core of the Earth cause
changes in the length of an Earth day -- the variations are about
one thousandth of a second over the course of a year.

Researchers say that lunar reflectors will remain in service
for years to come, because of the usefulness of continued
improvements in range determinations for further advancing our
understanding of the Earth-Moon system and the need for
monitoring the details of the Earth's rotation.

At JPL, this lunar ranging analysis, sponsored by NASA's
Office of Space Science, is conducted by Drs. James G. Williams,
Dale Boggs, J. Todd Ratcliff and Jean O. Dickey. JPL is a
division of the California Institute of Technology, Pasadena, CA.